If you have an android, go to the play store and download Scheduler for WhatsApp App on your smartphone.
Install the app on your Android phone and open it, you will get a dialogue box on your screen asking you about the accessibility settings. Just tap on OK and continue the further procedure.
The rest is easy. Enter contact name or phone number, then what time the message should be sent, then write the message and boom!
I’ll upload a version of that same app without ads soon.